Environmental noise pollution is generated by many sources including traffic on roads, airports, construction sites, electrical substations, factories and school playgrounds. Such noise can be very disturbing for humans and animals, and can cause both psychological and physiological reactions. For these reasons, legislation against noise pollution has been enacted in many jurisdictions. Compliance with environmental legislation has, therefore, become a key issue in the development of new technologies in many industries.
Currently, acoustic barriers are used to attenuate noise. Such acoustic barriers are generally designed to absorb, reflect or otherwise attenuate the acoustic waves generated by a source. Acoustic barriers can be grouped into three main types.
Passive acoustic barriers generally comprise walls made of, for example, solid masonry or concrete. Such walls can be expensive to construct, they are often required to be very thick, and they may not attenuate low-frequency acoustic waves (which can be particularly irritating to humans) effectively.
Reactive acoustic attenuator elements are also known to attenuate acoustic waves through resonance effects. Such elements are more effective for low-frequency sound attenuation, and may provide attenuation across a resonant frequency band.
Sonic crystals, which typically comprise multiple rows of acoustic attenuator elements arranged periodically, are also known to attenuate acoustic waves through diffraction effects. However, the provision of multiple rows of acoustic elements requires a significant land footprint, which can be expensive particularly in built up areas.
Reactive acoustic elements can be arranged together to form an acoustic barrier in the form of a macroscopic sonic crystal in order to achieve attenuation of acoustic waves over different frequency bands or a stronger attenuation over a particular frequency band. However, it would be beneficial to reduce the land footprint of acoustic barriers which utilise these effects.